Thin cathode for micro-battery

ABSTRACT

Methods of forming a battery include forming a thin graphene cathode on a substrate. A lithium anode is formed and an electrolyte is formed between the thin graphene cathode and the lithium anode.

BACKGROUND Technical Field

The present invention generally relates to batteries and, moreparticularly, to the use of a thin carbon cathode in lithium-oxygenbatteries.

Description of the Related Art

Lithium-ion batteries are prevalent in fields such as consumerelectronics, automobiles, medical devices, and home energy storage. In alithium ion insertion reaction, the number of lithium ions that can beinserted into a host cathode determines the amount of energy stored inthe battery. As a result, a large cathode is needed to increase thestorage capacity of the battery. There is therefore a limit to how smallan effective lithium ion battery can be made while providing a usefulenergy density.

Lithium-oxygen battery chemistries have a higher gravimetric andvolumetric energy density (e.g., about 3,213 Wh/kg and about 7,422 Wh/Lrespectively with respect to only cathode mass or volume) than one ofthe most commonly used cathode materials, LiCoO₂ (e.g., about 1,095Wh/kg and about 5,543 Wh/L respectively with respect to only cathodemass or volume). Lithium-oxygen batteries therefore present a pathtoward further miniaturization, decreasing the weight and volume ofbatteries without sacrificing energy capacity. However, existinglithium-oxygen battery implementations use large, porous cathodes thatstill impose a volumetric disadvantage for miniaturized applications.

SUMMARY

A method of forming a battery includes forming a thin graphene cathodeon a substrate. A lithium anode is formed and an electrolyte is formedbetween the thin graphene cathode and the lithium anode.

Another method of forming a battery includes forming a cathode on asubstrate from a single- or double-layer graphene material and forming alithium anode. An electrolyte having a high solubility for lithium ionsand oxygen between the cathode and the lithium anode is formed, suchthat lithium ions migrate from the anode through the electrolyte to formLi₂O₂ at a surface of the cathode. The method also includes positioninga current collector formed from a metal mesh in the electrolyte.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of a lithium-oxygen battery having athin graphene cathode in accordance with an embodiment of the presentinvention;

FIG. 2 is a block/flow diagram of a method of forming a lithium-oxygenbattery having a thin graphene cathode in accordance with an embodimentof the present invention; and

FIG. 3 is a graph illustrating the effect of using a thin graphenecathode in a lithium-oxygen battery as compared to the use of a baresubstrate in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention employ a thin cathode layer formedfrom, e.g., graphene with a thickness of a few atoms to formlithium-oxygen batteries to use as a nucleation seed for asolution-mediated lithium-oxygen battery discharge reaction. The presentembodiments thereby provide a cathode capacity of more than about 0.05mAh/cm² that is twice the cathode capacity of LiCoO₂ obtained with asimilar volume of LiCoO₂ to Li₂O₂. The present embodiments furthermoreprovide a gravimetric energy density per cathode mass that is twothousand times higher than that of LiCoO₂ and four times higher percathode mass when the weight of discharge products are included.

Referring now to FIG. 1, a cross-sectional view of a lithium-oxygenbattery 100 is shown. An anode 102 is separated from a cathode 108 by anelectrolyte 104 and separator 106. The electrolyte 104 provides aconductive channel for the movement of charge carriers from the anode102 to the cathode 108 during discharge reaction. The separator 106 is anon-conductive, porous structure that prevents the anode 102 and cathode108 from coming into electrical contact with one another. In someembodiments, the electrolyte 104 is a fluid. In other embodiments, theelectrolyte 104 is a solid material that also plays the role ofseparator 106. The cathode 108 is formed on a substrate layer 110 thatmay be flexible or rigid, conductive or non-conductive, flat or rough,and that is used to transfer the cathode 108.

In one specific embodiment, the anode 102 is formed from a layer oflithium metal, but it should be understood that other materials such as,e.g., sodium or other alkaline metals, may be used instead. In onespecific embodiment, the cathode 108 is formed from graphene, a verythin species of carbon that can be formed to a thickness of a singleatom. In one specific embodiment, the separator 106 may be a porouspolymeric film such as, e.g., polyethylene or polypropylene or quartz(SiO₂) microfiber filters, and may be formed at a thickness betweenabout 25 μm and 450 μm. In embodiments with mechanically strong,solid-state electrolytes (which can function as a separator) 104 may beas thin as hundreds of nanometers. In one specific embodiment, thesubstrate 110 may be formed from, e.g., a silicon or silicon dioxidewafer, a stainless steel pad, glass, or a polyimide film.

In one specific embodiment, the electrolyte 104 is formed from anappropriate liquid electrolyte material such as, e.g., a solution thathas LiNO₃ or (Lithium Bis(trifluoromethanesulfonyl)imide) (LiTFSI) assalts, mixed with 1,2-dimethoxyethane (DME) or tetraethylene glycoldimethyl ether (TEGDME) as solvents. The electrolyte enhances batterycapacity in single- and double-layer graphene cathodes and have a highsolubility of intermediate species (e.g., Li⁺ and O₂ ⁻) during theformation of Li₂O₂. Because the intermediate species can dissolve intothe electrolyte during discharge, those species can migrate farther toform larger particles of Li₂O₂, rather than precipitating into a filmand passivating the cathode surface. Such electrolytes may furtherinclude small amounts of water.

A current collector 107 is positioned in the electrolyte 104 and may, insome embodiments, be formed from stainless steel or titanium. It isspecifically contemplated that the current collector 107 may be formedfrom, e.g., a conductive wire mesh formed from any appropriate metal orother conductor that will not react with the electrolyte 104 orotherwise corrode. The current collector 107 may include a mesh that hasopenings smaller than about 38 μm. This sizing represents just oneexample—a finer mesh will provide better electron distribution. Thecurrent collector 107 leaves the battery 100 to connect to an externalcircuit.

It should be understood that the present embodiments illustrate only onepossible example of the use of a thin carbon cathode in a battery. It isspecifically contemplated that, in this embodiment, the cathode 108 maybe formed on a layer of copper by, e.g., chemical vapor deposition (CVD)or any other appropriate mechanism. For example, a layer of graphene maybe formed on the copper layer by carbon CVD, where the atoms of carbonself-organize into a flat sheet that is one or more atoms thick.

Some embodiments may employ the copper layer directly as the substrate110. However, experimental evidence has shown that a graphene/copperelectrode shows Li₂O₂ formation until 2.1V, followed by electrochemicalreactions from the copper at potentials below 2.1V. Thus, the presentembodiments transfer the graphene to an alternative substrate material.The cathode 108 may therefore be mounted to an intermediate handlinglayer formed from, e.g., poly(methyl methacrylate) (PMMA),ethylene-vinyl acetate (EVA), or any other appropriate material that hasetch selectivity with the copper layer. The copper layer is then etchedaway using, e.g., FeCl₃, allowing the cathode 108 to be moved to thesubstrate layer 110. The handling layer is then etched away using, e.g.,acetone for PMMA or xylene for EVA, and the substrate 110 and cathode108 can be affixed to the battery 100. This process for creating andtransporting the cathode 108 has an advantage in that it does not need ahigh-temperature anneal, the way lithium-ion cathodes do, and in thatthe cathode 108 can be transferred to non-conducting or conductingsurface. However, this illustrates just one possible process for forminga thin cathode layer—any other appropriate process can be used instead.In some embodiments, the cathode may be about 1 nm and about 2 nm thick.

During operation of a lithium-oxygen battery, lithium ions diffuseacross the electrolyte from the anode 102 to the cathode 108, where itreacts with oxygen at the cathode 108 and forms Li₂O₂. This movement ofpositive ions is accompanied by a flow of electrons in the currentcollector 107 toward the device 100, representing a discharging action.The Li₂O₂ accumulates on the surface of the cathode 108 during thisdischarging action.

Referring now to FIG. 2, a method of fabricating a battery is shown.Block 202 forms the cathode 108 on a first substrate. As noted above, itis specifically contemplated that the cathode 108 may be formed fromgraphene and may be very thin (e.g., between about 1 nm and 2 nm) andthat the first substrate may be, e.g., copper or nickel. The cathode 108may be formed by CVD or any other appropriate deposition process thatallows atoms of the cathode material (e.g., carbon) so self-organize onthe surface of the first substrate.

CVD is a deposition process in which a deposited species is formed as aresult of chemical reaction between gaseous reactants at greater thanroom temperature (e.g., from about 25° C. about 900° C.). The solidproduct of the reaction is deposited on the surface on which a film,coating, or layer of the solid product is to be formed. Variations ofCVD processes include, but are not limited to, Atmospheric Pressure CVD(APCVD), Low Pressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), andMetal-Organic CVD (MOCVD) and combinations thereof may also be employed.

Block 204 attaches a handling layer to the cathode layer 108. It isspecifically contemplated that the handling layer may be formed from,e.g., PMMA, but it should be understood that any material having etchselectivity with the first and second substrates and the cathode 108 maybe used instead. The handling layer may be applied by any appropriatemechanism including, e.g., spin coating. As used herein, the term“selective” in reference to a material removal process denotes that therate of material removal for a first material is greater than the rateof removal for at least another material of the structure to which thematerial removal process is being applied.

Block 206 then etches away the first substrate using an appropriate wetor dry etch such as, e.g., FeCl₃. The cathode 108 remains attached tothe handling layer and can be moved into position over a secondsubstrate (which forms substrate 110 of the finished battery 100). Thecathode 108 covered by the second substrate can optionally be patternedin block 207 using the handling layer as a photoresist. The cathodesurface that is not covered by the handling layer can be selectivelyremoved. In the example where the cathode 108 is formed from grapheneand the handling layer is formed from PMMA, the graphene cathode can beetched using an oxygen plasma. This is a simpler process than patterninginorganic cathodes formed from, e.g., LiCoO₂, LiFePO₄, orLiNi_(x)Mn_(y)Co_(z)O₂ because they require harsh chemical etchingmethods. Building arrays of batteries and the accompanying circuitdesign is similarly simplified by simple micropatterning of graphenebased cathodes, in particular for micron scale devices.

The cathode 108 is attached to the second substrate in block 208. Noadhesive may be needed to attach the cathode 108 to the secondsubstrate—instead an attractive force is present due to, e.g., Van derWaals interactions. The handling layer is then etched away using anyappropriate wet or dry etchant such as, e.g., acetone, leaving thecathode 108 on the substrate 110.

Block 212 adds a current carrier 107 in the form of a conductive mesh ontop of the cathode layer 108. Block 214 adds a separator 106 on thecurrent carrier 107, covering the current carrier 107 and the cathodelayer 108. The separator may be formed from, e.g., a thin sheet ofporous membrane. Block 216 adds anode 102 on the separator 106. Theanode 102 is formed from an appropriate metal such as, e.g., lithium,LiC₆, Li₇Ti₅O₁₂, or Li_(4.4)Si. Block 218 then introduces a liquidelectrolyte between the anode 102 and the cathode 108. It isspecifically contemplated that the electrolyte may be formed from LiNO₃or LiTFSI as salts mixed in an ether-based solvent (e.g., DME, TEGDME),but any appropriate electrolyte composition may be used instead.

It is to be understood that aspects of the present invention will bedescribed in terms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps can be varied within the scope of aspects of the presentinvention.

Referring now to FIG. 3, a graph of the relationship between cellvoltage on the vertical axis 302, measured in Volts, and charge capacityon the horizontal axis 304, measured in mAh/cm² at the current densityof 4 μA/cm², is shown for two different cathode materials in a DME-basedelectrolyte mixed with LiNO₃ to facilitate solution-mediatedlithium-oxygen battery discharge reactions. A bare silicon or silicondioxide layer is shown by curve 308 and a graphene layer mounted on asilicon or silicon dioxide layer is shown by curve 306. Those cathodeswere tested against lithium metal as anode. A flat voltage profile isshown around 2.5V for curve 306, but no significant discharge capacityis provided by the bare wafer. Similar results are provided byelectrolyte solutions based on TEGDME.

The discharge products of the discharge reaction are confirmed to beLi₂O₂ by measuring the Raman spectrum (e.g., using a 532 nm laser) ofpristine and discharged graphene cathodes. A peak at about 790 cm⁻¹appears after discharge, corresponding to O—O bond stretching in Li₂O₂.The growth of Li₂O₂ is enabled by having a single/double layer ofgraphene. The graphene surface can function as a seed for Li₂O₂ growtheven in the presence of a non-conducting substrate 110. With a TEGDMEelectrolyte and a 0.5M LiNO₃ solution the present embodiments canprovide about 60 μAh/cm² of capacity at 24 μA/cm² of current density.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements can also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements can be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments can include a design for an integrated circuitchip, which can be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer can transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1−x) where x is less than or equal to 1, etc. Inaddition, other elements can be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment”,as well as other variations thereof, means that a particular feature,structure, characteristic, and so forth described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof the phrase “in one embodiment” or “in an embodiment”, as well anyother variations, appearing in various places throughout thespecification are not necessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This can be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including, ” when usedherein, specify the presence of stated features, integers steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGS. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, if the device in theFIGS. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein can be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers can also be present.

It will be understood that, although the terms first, second, etc. canbe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A method of forming a battery, comprising:forming a thin graphene cathode on a substrate; forming a lithium anode;and forming an electrolyte between the thin graphene cathode and thelithium anode.
 2. The method of claim 1, wherein the thin graphenecathode is a layer selected from the group consisting of single-layergraphene and double-layer graphene.
 3. The method of claim 1, whereinthe electrolyte comprises 1,2-dimethoxyethane.
 4. The method of claim 1,wherein the electrolyte comprises tetraethylene glycol dimethyl ether.5. The method of claim 1, wherein the electrolyte comprises a soluteselected from the group consisting of LiNO₃ and (LithiumBis(trifluoromethanesulfonyl)imide).
 6. The method of claim 1, furthercomprising positioning a current collector in the electrolyte betweenthe cathode and a separator.
 7. The method of claim 6, wherein thecurrent collector is a metal mesh formed from a material selected fromthe group consisting of stainless steel and titanium.
 8. The method ofclaim 7, wherein the metal mesh has openings that are smaller than about38 μm.
 9. The method of claim 1, wherein forming the thin graphenecathode comprises: depositing the thin graphene layer on an initialsurface; transferring the thin graphene layer from the initial surfaceto an intermediate surface; and transferring the thin graphene layerfrom the intermediate surface to the substrate.
 10. The method of claim9, further comprising patterning the thin graphene layer aftertransferring the thin graphene layer to the intermediate surface, usingthe intermediate surface as a mask.
 11. The method of claim 1, whereinthe electrolyte is a fluid through which lithium ions migrate from theanode to form Li₂O₂ at a surface of the cathode, further comprisingconnecting the current collector to an external circuit.
 12. A method offorming a battery, comprising: forming a cathode on a substrate from asingle- or double-layer graphene material; forming a lithium anode;forming an electrolyte having a high solubility for lithium ions andoxygen between the cathode and the lithium anode, such that lithium ionsmigrate from the anode through the electrolyte to form Li₂O₂ at asurface of the cathode; and positioning a current collector formed froma metal mesh in the electrolyte.
 13. The method of claim 12, wherein theelectrolyte comprises 1,2-dimethoxyethane.
 14. The method of claim 12,wherein the electrolyte comprises tetraethylene glycol dimethyl ether.15. The method of claim 12, wherein the electrolyte comprises a soluteselected from the group consisting of LiNO₃ and (LithiumBis(trifluoromethanesulfonyl)imide).
 16. The method of claim 12, whereinthe substrate is flexible.
 17. The method of claim 12, wherein thecurrent collector is a metal mesh formed from a material selected fromthe group consisting of stainless steel and titanium and wherein themetal mesh has openings that are smaller than about 38 μm.
 18. Themethod of claim 12, wherein forming the thin graphene cathode comprises:depositing the thin graphene layer on an initial surface; transferringthe thin graphene layer from the initial surface to an intermediatesurface; and transferring the thin graphene layer from the intermediatesurface to the substrate.
 19. The method of claim 18, further comprisingpatterning the thin graphene layer after transferring the thin graphenelayer to the intermediate surface, using the intermediate surface as amask.
 20. The method of claim 18, wherein the initial surface is formedfrom copper and wherein the intermediate surface is formed from amaterial selected from the group consisting of poly(methyl methacrylate)and ethylene-vinyl acetate.